When new, gain and offset of exhaust gas sensors such as NOx sensors may be determined in order to calibrate the sensor so that accurate measurements of exhaust gas constituents may be obtained. Over time, however, sensor parameters can drift resulting in measurement error. For example, a sensor gain and offset, which are calibration parameters used to define a linear transformation from sensor output to a measured parameter, may drift during sensor operation. Thus, periodic updates of the sensor parameters, including sensor gain and offset, during the lifetime of the sensor may be used.
One approach to periodically update NOx sensor offset is disclosed in U.S. Pat. No. 6,311,480. In the cited reference, a deviation of an output value (e.g., an offset) of a NOx sensor from a reference output value (e.g., NOx concentration of ambient air) is determined when the engine is under selected operating conditions which may include engine idling, low-load operation, and fuel cut operation. During such operating conditions, NOx concentration in the exhaust gases in the vicinity of the sensor decreases to a value that is substantially zero (e.g., NOx concentration of ambient air). As such, a deviation of the output value of the NOx sensor from an output value corresponding to zero NOx concentration corresponds to a drift in the offset of the sensor, and subsequent NOx readings may be adjusted based on the offset. Thus, accurate NOx readings may be obtained despite offset deviations.
The location of the NOx sensor, however, may affect NOx concentration during engine operating conditions described above, such as during fuel cut operation (e.g., deceleration fuel shut off, DFSO). For example, if the sensor is located downstream of a NOx storage catalyst, exhaust gases reach the sensor only after exiting the catalyst. There is a chance NOx may exit the catalyst in the exhaust stream in some situations including when the catalyst has not warmed up and when the catalyst has degraded. In such a situation, the approach disclosed in the above reference would yield an inaccurate NOx offset which may lead to an increase in NOx emissions.
The inventor herein has recognized the above problems and has devised an approach to address them. Thus, a method for generating an offset of a NOx sensor downstream of a catalyst is disclosed. The method comprises generating an offset of the sensor based on an ambient air flow drawn into the exhaust system, the ambient airflow having circumvented cylinders of the engine, and adjusting an output of the sensor based on the generated offset.
Specifically, in one example, an ambient ram air flow that is absent of exhaust emissions is selectively drawn into the exhaust system through a valved bypass and the ram air flows past the NOx sensor. In this manner, an accurate NOx offset of the sensor may be periodically generated in order to maintain and/or reduce NOx emissions, even if the NOx sensor is positioned downstream of a catalyst.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.